In many applications, typically in various communications systems and especially in multi-carrier and multi-user systems, there are requests for non-linear modification of a signal. In many cases, such non-linear modifications have to be kept within a certain bandwidth or within certain spectral mask restrictions. One typical example of such non-linear modification is Peak-to-average ratio (PAR) reduction, also known as decresting or clipping. PAR reduction increases efficiency and average output power of a power amplifier (PA). The objective is to reduce the peak amplitude excursions of the output signal while keeping the spectrum expansion within specified limits, such as spectral mask and adjacent channel power ratio (ACPR) specifications, and keeping in-band error within specified limits, so-called error vector magnitude (EVM) specification.
Existing prior art solutions are mainly found in the PAR reduction area. Many different approaches have been suggested. Some locate a local amplitude maximum and insert a band-limited pulse to suppress it. Some predict the overshoot from filtering of the clipped signal and pre-compensate for it.
In the published international patent application WO 03/001697 a PAR system is disclosed. An error signal corresponding to crests of the input signal is generated and subtracted from the input signal. It is also suggested to implement multiple steps of decresting sequentially. The basic idea is very attractive and such systems do really reduce the PAR, but to achieve a good performance meeting EVM and ACPR specifications, a large number of steps have to be used. This increases the complexity of the implementation and therefore also the costs.